Field of the Invention
The present invention relates to a resin-coated carrier for an electrophotographic developer used in a two-component electrophotographic developer for use in apparatuses such as copiers and printers, and an electrophotographic developer using the resin-coated carrier.
Description of Related Art
An electrophotographic development method is a method in which development is performed by adhering the toner particles in a developer to the electrostatic latent image formed on a photo conductor, and the developer used in such a method is classified into a two-component developer composed of toner particles and carrier particles and a one-component developer using only toner particles.
As a development method using, among such developers, a two-component developer composed of toner particles and carrier particles, previously a method such as a cascade method has been adopted, but currently a magnetic brush method using a magnet roll predominates.
In a two-component developer, the carrier particles serve as a carrying substance to form a toner image on the photo conductor in such a way that the carrier particles are stirred together with the toner particles in a developer box filled with the developer to impart an intended charge to the toner particles, and further, convey the thus charged toner particles to the surface of the photo conductor to form the toner image on the photo conductor. The carrier particles remaining on a development roll which holds a magnet again return from the development roll into the developer box to be mixed under stirring with the fresh toner particles and to be repeatedly used for a predetermined period of time.
In contrast to a one-component developer, a two-component developer is such that the carrier particles are mixed under stirring with the toner particles, thus charge the toner particles, and further have a function to convey the toner particles, and a two-component developer is excellent in the controllability in designing developers. Accordingly, two-component developers are suitable for full-color development apparatuses required to offer high image quality and for high speed printing apparatuses required to be satisfactory in the reliability and durability in image maintenance.
In two-component developers used in the above-described manner, the image properties such as the image density, fogging, white spots, gradation and resolution are each required to exhibit a predetermined value from the initial stage, and further these properties are required to be invariant and to be stably maintained during the endurance printing. For the purpose of stably maintaining these properties, the properties of the carrier particles contained in the two-component developers are required to be stable.
As the carrier particles which form two-component developers, there have hitherto been used various types of iron powder carriers, ferrite carriers, resin-coated ferrite carriers, magnetic powder-dispersed resin carriers and the like.
Recently, office networking has been promoted, and the age of monofunctional copiers develops into the age of multifunctional copiers; the service system has also shifted from the age of the system such that a contracted service man conducts periodic maintenance inclusive of the replacement of the developer to the age of the maintenance-free system; thus, the market has further enhanced demand for further longer operating life of the developer.
Under such circumstances, for the purpose of reducing the carrier particle weight and extending the developer operating life, there have been proposed in Japanese Patent Laid-Open No. 5-40367 and the like a variety of magnetic powder-dispersed carriers in each of which magnetic fine particles are dispersed in a resin.
Such magnetic powder-dispersed carriers can be reduced in true density by decreasing the amounts of the magnetic fine particles and can be reduced in stress caused by stirring, and hence can be prevented from the abrasion and exfoliation of the coating film and accordingly can offer stable image properties over a long period of time.
However, the magnetic powder-dispersed carrier is prepared by agglomerating magnetic fine particles with a binder resin, and hence offers, as the case may be, a problem that the magnetic fine particles are detached due to the stirring stress or the impact in the developing device or a problem that the carrier particles themselves are cracked probably because the magnetic powder-dispersed carriers are inferior in mechanical strength to the iron powder carriers and ferrite carriers having hitherto been used. The detached magnetic fine particles and the cracked carrier particles adhere to the photo conductor to cause image defects as the case may be.
Additionally, the magnetic powder-dispersed carrier uses magnetic fine particles, and accordingly has a drawback that the residual magnetization and the coercive force are high and the fluidity of the developer is degraded. In particular, when a magnetic brush is formed on a magnet roll, the presence of the residual magnetization and the coercive force hardens the ears of the magnetic brush and hence high image quality is hardly obtained. Additionally, even when the magnetic powder-dispersed carrier is separated away from the magnet roll, the magnetic coagulation of the carrier is not unstiffened and the mixing of the carrier with the refilled toner is not rapidly conducted, and hence there occurs a problem that the charge amount rise is aggravated, and image defects such as toner scattering and fogging are caused.
In addition to such a magnetic powder-dispersed carrier, for the purpose of reducing the weight of the carrier particle, there have been proposed hollow carriers in which a vacancy is formed in the interior of the carrier core material particle. For example, Japanese Patent Laid-Open No. 2008-310104 states that the core particle has at least one vacancy of 20% or more and 60% or less with reference to the cross-sectional area, and the total porosity is 20% or more and 70% or less with reference to the cross-sectional area. Japanese Patent Laid-Open No. 2009-244572 states that when the outer diameter of the carrier core material is represented by d1 and the outer diameter of the hollow portion present in the interior of the core material is represented by d2, it is preferable for the relation 0.1<d2/d1<0.9 to be satisfied.
In the carriers described in these patent documents, the weight reduction is certainly attained; however, in any of these carriers, the size of one vacancy is extremely large, and hence, as compared to conventional ferrite carriers having no hollow portion, these carriers are still weak in mechanical strength, thus the fracture of the carrier particles occurs due to the stirring stress or the impact in the developing device at the time of endurance printing, and the fractured particles adhere to the photo conductor to offer a cause for the occurrence of image defects. Accordingly, for the extension of the operating life having been recently, particularly demanded, these carriers are absolutely unsatisfactory.
Further, as the substitutes for such magnetic powder-dispersed carriers and hollow carriers, resin-filled carriers obtained by filling a resin in the voids of porous carrier core material have been proposed.
Japanese Patent Laid-Open No. 2006-337579 proposes a resin-filled carrier prepared by filling a resin in a ferrite core material having a porosity of 10 to 60%, and Japanese Patent Laid-Open No. 2007-57943 proposes a resin-filled carrier having a three-dimensional laminated structure. Further, Japanese Patent Laid-Open No. 2009-175666 proposes a resin-filled carrier which has a high dielectric breakdown voltage and has improved fracture strength of the carrier particle, through the specification of the pore volume, the pore size and the pore size distribution property of the porous ferrite core material to be filled with a resin.
In each of the resin-filled carriers described in these patent documents, a resin is filled even in the interior of the porous ferrite particles to form a three-dimensional laminated structure. In particular, in Japanese Patent Laid-Open No. 2009-175666, the pore size distribution property is controlled more accurately, and hence the variation of the resin filling degree is reduced, and further, it is stated that the surface of the filling resin is preferably coated with a resin. Therefore, indeed the weight reduction of the carrier particle is attained, and the carrier strength is improved; however, the carrier particle itself is made to have a high resistance and further made to have a three-dimensional structure composed of the resin and the ferrite, and hence a capacitor-like function works to cause a problem that the charge amount is increased at the time of endurance printing due to the accumulation of the charge, to make it difficult to ensure the image density and to obtain images high in image quality. Accordingly, for the extension of the operating life having been recently demanded, these carriers are far from being satisfactory, in a manner similar to hollow carriers.
On the other hand, achieved weight reduction makes magnetization per a carrier particle lower and as a result, problems such as defective image caused by scattering of a carrier from a magnetic brush to a photo conductor, i.e. easy generation of a carrier beads carry over has been arisen.
Therefore, with respect to the demand for extension of operating life, demanded is a carrier for an electrophotographic developer in which, while achieving the weight reduction, preventing the image defects such as a carrier beads carry over, further the carrier strength is being attained, and additionally the charging property is stabilized at the time of endurance printing.
As described above, there has been demanded a carrier for an electrophotographic developer in which, while the weight reduction is being attained, the defective image such as a carrier beads carry over is prevented and further the carrier strength is high, and additionally the charge amount variation is small at the time of endurance printing.